The present invention relates to a light emitting device using a group III nitride group compound semiconductor. In particular, the present invention relates to a light emitting device using a group III nitride group compound semiconductor in which a stack is formed on an upper surface of a group III nitride group compound semiconductor layer by epitaxial lateral overgrowth (ELO). The group III nitride group compound semiconductor layer comprises regions with many defects and regions with less defects. A group III nitride compound semiconductor can be made of binary compounds such as AlN, GaN or InN, ternary compounds such as AlxGa1xe2x88x92xN, AlxIn1xe2x88x92xN or GaxIn1xe2x88x92xN where (0 less than x less than 1), or quaternary compounds AlxGayIn1xe2x88x92xxe2x88x92yN where (0 less than x less than 1, 0 less than y less than 1, 0 less than x+y less than 1), that is, those are represented by a general formula AlxGayIn1xe2x88x92xxe2x88x92yN where (0xe2x89xa6xxe2x89xa61,0xe2x89xa6yxe2x89xa61,0xe2x89xa6x+yxe2x89xa61). In accordance with present invention, a group III nitride group compound semiconductor includes a group III nitride group compound semiconductor which is doped with impurities to have p-type or n-type conductivity.
A group III nitride group compound semiconductor is a direct-transition-type semiconductor having a wide emission spectrum range from ultraviolet to red, and is applied to light-emitting devices such as light-emitting diodes (LEDs) and laser diodes. The group III nitride group compound semiconductor is, in general, formed on a sapphire substrate. A laser diode, in general, comprises a guide layer and a cladding layer, which are formed on an n-type and a p-type semiconductor side of an active layer, respectively, sandwiching the same. The cladding layer is formed to have a large band gap and is generally made of AlxGa1xe2x88x92xN where (0 less than x less than 1) including aluminum (Al), such that electrons and holes injected from negative and positive electrodes generate electron-hole pairs in the active layer. The guide layer has a little wider band gap than the active layer. The guide layer is made of, for example, gallium nitride (GaN) such that laser lights can be confined in the active layer by difference of refractive indices. The active layer preferably has a multiple quantum well (MQW) structure.
FIG. 6 illustrates the structure of a laser diode 900 having a conventional group III nitride group compound semiconductor light-emitting device. The laser diode 900 comprises a saphire substrate 91, and an AlN buffer layer 92 formed thereon.
On the buffer layer 92, four layers are formed successively: an n-layer 93 made of silicon (Si) doped GaN; an n-cladding layer 94 made of silicon (Si) doped Al0.08Ga0.92N; an n-guide layer 95 made of silicon (Si) doped GaN; and an active layer 96 having a multiple quantum well (MQW) structure in which a barrier layer made of GaN and a well layer made of Ga0.85In0.15N are laminated together. On the active layer 96, a p-guide layer 97 made of magnesium (Mg) doped GaN, a p-cladding layer 98 made of magnesium (Mg) doped Al0.08Ga0.92N, and a p-contact layer 99 made of magnesium (Mg) doped GaN are formed. An electrode 910 is formed on the p-contact layer 99 and another electrode 911 is formed on a portion of the n-layer 93.
FIG. 7 is a schematic view of the laser diode 900. Rd represents a stack and Mrr represents a stack facet. Generally the stacks are formed by etching. The stack facets of several adjacent stacks form a cavity.
In the above-described conventional technique, however, when a layer of a group III nitride group compound semiconductor is formed on a sapphire substrate, dislocations are generated in the semiconductor layer due to a misfit between lattice constants of sapphire and the group III nitride compound semiconductor, which results in degraded device characteristics. In particular, the dislocations due to the misfit are feedthrough dislocations which penetrate the semiconductor layer in a longitudinal direction (a direction vertical to the surface of the substrate), resulting in propagation of about 109 cmxe2x88x922 of dislocation in the group III nitride group compound semiconductor. The dislocations are then propagated to the uppermost layer of the group III nitride group compound semiconductor layers each having different composition. When stack facets Mrr in FIG. 7 are formed by etching, ruggedness shown by xcfx86 in FIG. 8 is generated on the stack facets Mrr due to feedthrough dislocations. The ruggedness xcfx86 is about 20 nm in depth and formed in cylindrical pattern. Accordingly, the stack facets of the conventional laser diode 900 are remarkably far from ideal stack facets, which have a specular surface having no ruggedness. As a result, the oscillation efficiency of laser reflection becomes remarkably worse.
It is an object of the present invention to provide a light emitting device that overcomes the above-identified deficiencies.
It is another object of the present invention to provide a light emitting device using a group III nitride group compound semiconductor which comprises stack or resonator facets having less ruggedness.
It is another object of the present invention to provide a light emitting device having a group III nitride group compound semiconductor layer having a plurality of distinct regions having many defects and a plurality of distinct regions having less defects.
It is another object of the present invention to provide a light emitting device having a group III nitride group compound semiconductor layer having a plurality of distinct regions having many defects and a plurality of distinct regions having less defects, wherein the stack facets of a stack are arranged in an area having less defects.
It is another object of the present invention to provide a light emitting device using a group III nitride group compound semiconductor that efficiently suppreses feedthrough dislocations that are transmitted to the substrate in vertical direction.
These and other objects of the present invention will apparent in view of the description of the present invention and claims set forth below.
The present invention is directed to a light emitting device using a group III nitride group compound semiconductor comprising a group III nitride group compound semiconductor and a stack which is formed on the upper surface of a group III nitride group compound semiconductor layer comprising regions with many defects or less defects. The stack is formed so as to traverse the regions of the group III nitride group compound semiconductor layer with many defects and less defects and a stack facet is formed on the region of the group III nitride group compound semiconductor layer with less defects. The stack is formed by employing a process, e.g., cleaving or etching the laminated group III nitride group compound semiconductor layer.
In accordance with the present invention, the light emitting device may include a plurality of stacks which form cavities. The stacks may be formed by etching. It is contemplated that the light emitting device may be a laser diode or a light emitting diode.
In accordance with the present invention, the regions of the group III nitride group compound semiconductor layer with many defects and those regions with less defects are formed in a striped pattern at least near the stack facets. Here a striped pattern does not necessarily represents a rectangular with a short edge and a long edge. It is adequate if each boundaries between the regions of the group III nitride group compound semiconductor with many defects and less defects, which are placed near the stack facets, is a straight line and almost parallel to each other. Boundaries may not be necessarily observed by an apparatus but may be recognizable in a manufacturing process as a divided line that divides the regions with many defects and less defects.
In accordance with the present invention, the stack facets may be parallel to the boundaries between the regions of the group III nitride group compound semiconductor with many defects and less defects, which are placed near the stack facets. Boundaries are used to distinguish the group III nitride group compound semiconductor layer. For example, when the group III nitride group compound semiconductor are formed on the substrate, the boundaries are vertical to the surface of the substrate. Boundaries may not be necessarily observed by an apparatus but may be recognizable in a manufacturing process in order that regions with many defects and less defects exist divided by the boundaries. And a boundary surface may not necessarily correspond to a growing facet when the group III nitride group compound semiconductor layer grows by epitaxial growth.
In accordance with the present invention, each boundary between the regions of the group III nitride group compound semiconductor with many defects and less defects, which are placed near the stack facets, is a {11-20} surface of the group III nitride group compound semiconductor. Boundaries are used to divide the region of the group III nitride group compound semiconductor layer. For example, when the group III nitride group compound semiconductor are formed on the substrate, the boundaries are vertical to the surface of the substrate. Boundaries may not be necessarily observed by an apparatus but may be recognizable in a manufacturing process so that regions with many defects and less defects can be proved to exist divided by the boundaries.
In accordance with the present invention, at least a bottom layer which exists in the regions of the group III nitride group compound semiconductor with less defects is formed by epitaxial lateral overgrowth (ELO). Epitaxial lateral overgrowth (ELO) represents, for example, an epitaxial growth in a direction parallel to the surface of the substrate when the group III nitride group compound semiconductor is formed on the substrate. The group III nitride group compound semiconductor may also grow longitudinal direction (a normal direction of the surface of the substrate) epitaxially accompanying with ELO.
In accordance with another aspect of the present invention, at least a bottom layer which exists in the regions of the group III nitride group compound semiconductor with less defects is a group III nitride group compound semiconductor layer whose growing facet by epitaxial lateral overgrowth (ELO) is a {11-20} surface, or a group III nitride group compound semiconductor layer which is formed on the group III nitride group compound semiconductor layer grown by epitaxial lateral overgrowth (ELO) with a growing facet of {11xe2x88x9220} surface. The group III nitride group compound semiconductor layer comprising regions with many defects or less defects is grown by epitaxial lateral overgrowth (ELO) as a growing facet of {11-20}. This does not necessarily exclude the possibility that the group III nitride group compound semiconductor layer grows epitaxially in longitudinal direction while it grows epitaxially in lateral direction. The group III nitride group compound semiconductor layer may be grown epitaxially not only in lateral direction but also in longitudinal direction. Also, the second group III nitride group compound semiconductor layer is formed on the first group III nitride group compound semiconductor layer which is grown by epitaxial lateral overgrowth (ELO). This does not necessarily exclude the possibility that a first group III nitride group compound semiconductor layer grows epitaxially in longitudinal direction while it grows epitaxially in lateral direction. The first group III nitride group compound semiconductor layer may be grown epitaxially not only in lateral direction but also in longitudinal direction. Compositions of the second group III nitride group compound semiconductor layer which is formed on the first group III nitride group compound semiconductor layer and the first group III nitride group compound semiconductor layer, respectively, can be same or different. Also, impurities doped into the first and the second group III nitride group compound semiconductors, respectively, and their doping amount can be same or different. Further, an arbitrary number of group III nitride group compound semiconductor layer can be laminated on the first group III nitride group compound semiconductor layer which is grown by epitaxial lateral overgrowth (ELO).
A light emitting device using a group III nitride group compound semiconductor comprises a stack. The stack is formed so as to traverse regions of a group III nitride group compound semiconductor layer with many defects or less defects and a stack facet is formed on the region of the group III nitride group compound semiconductor layer with less defects. As a result, only by forming the regions with less defects extremely small, ruggedness of the stack facet formed in the laser diode using group III nitride group compound semiconductor can be suppressed. The ruggedness is suppressed remarkably when the stack, especially the stack facet, is formed by etching. The group III nitride group compound semiconductor layer comprises regions with many defects or less defects formed in a striped pattern at least near the stack facets. The size of the region formed in a striped pattern is sufficient if the stack facets can be formed thereon. By forming the stack facets to be parallel to boundaries between the regions of the group III nitride group compound semiconductor layer with many defects and less defects, which are placed near the stack facets, the region with less defects can be formed smaller. When each boundaries between the regions of the group III nitride group compound semiconductor layer with many defects and less defects, which are placed near the stack facets, is a {11-20} surface of the group III nitride group compound semiconductor layer, etching process can be carried out easier by adjusting the stack facets to be the {11xe2x88x9220} surface, resulting in suppressing the ruggedness on the surface of the group III nitride group compound semiconductor layer.
The group III nitride group compound semiconductor layer comprising regions with many defects and less defects can be easily formed by epitaxial lateral overgrowth (ELO). Especially the growing facet of the group III nitride group compound semiconductor layer by ELO is adjusted to be the {11-20} surface of the group III nitride group compound semiconductor layer. In this case the growing facet of the group III nitride group compound semiconductor layer growing epitaxially in lateral direction becomes vertical to the substrate. As a result, feedthrough dislocations that are transmitted to the substrate in vertical direction can be suppressed efficiently.